This invention relates in general to techniques for non-destructive characterization, identification and mapping of atoms and/or molecules adsorbed on solid surfaces, and other surface features. In particular, it relates to such techniques employing difference frequency generation with a scanning tunneling microscope or similar type of instrument.
With the advent of instruments such as the scanning tunneling microscope (STM), it is now possible to investigate the structure, spectra and dynamics of atoms, molecules and surfaces at the atomic or molecular level. For examples of non-linear alternating current STM techniques, please see U.S. Pat. Nos. 5,268,573 and 5,281,814.
In the two patents referenced above, a microwave sweep oscillator is used to apply an AC signal across a STM and a current or voltage passing between the electrodes of the STM is measured by a microwave spectrum/network analyzer. The frequency of the signal applied by the oscillator is swept across the spectrum and the optimum frequency of the spectrum is determined so that an improved image of the surface of a sample is obtained. The spectrum of a known substance is recorded and used as a signature for identifying components of an unknown substance by comparison. When the amplitude of the AC signal applied is increased, a sudden change in response indicates a threshold. The scheme is used for indicating charge dissipation characteristics or dynamic information of an electrochemical reaction.
Scanning tunneling microscopy and atomic force microscopy can indicate the presence of a single feature, but typically cannot determine the feature's identity. In the area of molecular structure determination, two techniques are most common. The first is X-ray diffraction, which requires crystals of the material to be studied, which is a severe limitation. The second is nuclear magnetic resonance, which remains complicated for large molecules and also requires a sample containing many molecules. It is therefore desirable to provide a system where single atoms, molecules or sub-micron surface features can be characterized and identified.